1. Field of the Invention
This invention relates to heating or annealing of silicon nitride based material in the semiconductor art such as in integrated circuits.
2. Prior Art
In the manufacture of miniature electronic devices such as semiconductor integrated circuits including metal oxide semiconductors (MOS), it is frequently desired to establish electrical interconnections between two parts of the device by means of a conductive film making contact to the parts to be interconnected. Typically, this conducting film has a portion that overlies an insulating material layer and makes contact though small apertures in the insulator (e.g. silicon dioxide) to the underlying device portion (e.g. silicon). In addition, it is often desirable that this conducting film cross other films, which might be conducting, insulating or semiconducting. To accomplish this, conductive film (e.g. aluminum) is vacuum evaporated or otherwise deposited atop the device structure and photoengraved to leave a desired pattern of conductors.
The sequential film-forming and photoengraving processes utilized to construct the underlying device structure generally result in the occurrence of variations of height, comparable to the thicknesses of the films involved. Certain of these changes in surface elevation can have very steep, or even overhanging edges. These edges can act as stress-concentrating regions and can result in occurrence of cracks in the conducting film that must traverse them. Such cracks are extremely deleterious. They can cause low production yield and can result in products that have high rates of failure in use.
One solution of this problem (U.S. Pat. No. 3,825,442--Moore) is to place the device in a furnace for a predetermined period of time at an elevated temperature until the insulating material flows to form a smooth surface topography around the apertures. Extreme care must be taken to avoid having the whole device heated to high temperatures which can cause migration of dopants, alloying, contaminating or other compositional charges.
An improved solution proposed by applicant in continuation application U.S. Ser. No. 339,600 filed Jan. 15, 1982 has been to employ a laser beam to induce densification or flow of phosphosilicate glass used as insulator layers in semiconductor devices. This process utilizes the absorption coupling of a laser beam tuned to selectively excite and thus preferential heat the SiO.sub.2 -based layer rather than surrounding strata of silicon substrate and interconnect regions. U.S. Pat. No. 4,284,659 (Jaccodine et al.) also disclose high energy laser radiation selectively coupled to a glass dielectric layer i.e. phosphorous doped silicon glass, deposited by the well-known chemical vapor deposition process. A continuous wave CO.sub.2 laser having a wavelength of 10.6 .mu.m with a dwell time on the layer being worked of 1 msec is employed. U.S. Pat. No. 4,316,074 (Daly) shows a method and apparatus and discusses other prior art with respect to each annealing of semiconductor materials. It includes much on the physics and theory of the operation, involving primarily a Nd:YAG (neodymium-doped yttrium aluminum garnet laser in the cw-pumped, Q-switched configuration usable in a scanning mode.
Also various semiconductor devices include the use of silicon nitride (Si.sub.3 N.sub.4) layers and regions for the purpose of passivation, isolation, gate dielectric, high dielectric contact capacitors, etc. Since the flow and annealing temperature of Si.sub.3 N.sub.4 is greater than SiO.sub.2 the aforementioned furnace annealing process cannot be utilized except at temperatures of at least 950.degree. C. for periods of about 30 minutes to aneal, densify or to induce compositional changes in Si.sub.3 N.sub.4, in the presence of other integrated circuit device materials such as silicon, silicon dioxide and aluminum. Flow requires temperatures exceeding 1200.degree. C. These temperatures and times may well cause dopant migration in the other layers and melting of aluminum layers which have a melting point of 660.degree. C., or which will alloy with silicon at 577.degree. C. The laser disclosed in the '659 patent has no utility for silicon nitride treatment. Accordingly, there is a need to develop a selective heating process for coupling radiation to Si.sub.3 N.sub.4 layers in semiconductor devices which can effectively treat silicon nitride at elevated temperatures to anneal, flow, densify and induce compositional changes, with minimal heat conduction into other portions of the structure, such as the surrounding or underlying layers.